The present invention relates to high resolution digital imaging and in particular to a system and method for correcting curvilinear distortion in digital images captured using optical lenses.
When images are captured using digital devices, such as a digital charge coupled device (CCD) camera used in x-ray imaging, an optical lens (or lens assembly) is used to focus the image onto the surface of the device. Whenever a lens (or a lens assembly) is used, the image suffers from distortion which causes the image to increasingly shrink in size as it proceeds outwards from the center because of the curvilinear nature of the lens. This type of optical distortion is referred to as barrel distortion as the straight edges of a rectangle appear to be curved in the shape of a barrel.
There are many applications where the use of non-distorted digital images is desirable. For example, a digitally captured x-ray image may be used to measure the distance between two vertebrae in the spine of an individual. In such applications, it is desirable to have a non-distorted image, so that a care provider can accurately measure the distance between the vertebrae and possibly observe the change in the distance of the vertebrae over a period of time. In a digital image having the distortion as discussed above, it is not possible to get such desirable accurate images. Thus, it is desirable to remove barrel distortion in order that accurate measurements may be made directly from a captured image.
Another problem with barrel distortion occurs when two images of different portions of the same object are to be stitched or joined together. Whenever barrel distortion is present, the images cannot be overlapped to produce a final seamless image. If the images are curved at the corners as discussed above, then it is very difficult to accurately combine or join the images to provide a combined or complete image. Thus, it is also desirable to remove barrel distortion whenever two or more images of the same object are to be stitched or combined together to form a single image.
Therefore, in one camera systems, it is desirable to correct optical distortion to provide images which may be used for different purposes, for example, making accurate measurements and in multi-camera systems, it may be desirable to correct optical distortion so that the different images may be combined together to form a seamless combined image.
Moreover, in many cases, the barrel distortion may not be symmetric. This may occur when the lens used is not symmetric. It may also occur when two or more lens (or lens assemblies) which are not perfectly aligned within the field of view are used. When barrel distortion is not symmetric, the correction depends upon the rotation of the device relative to the imaging surface.
Existing barrel correction methods are very computation intensive and require a lot of processing time to correct optical distortion. Such algorithms may not be suitable in various applications especially those applications which require multiple images to be acquired in a short period of time. For example, in diagnostic x-ray imaging systems, it may be desirable to acquire multiple images in a short period of time in order to reduce the patient""s exposure to x-ray radiation.
Therefore, there is a need in the art for a system and method for accurately and efficiently correcting optical distortion in digital images.
These and other objects, features and technical advantages are achieved by an imaging system and method which utilizes a calibration image to correct for optical distortion, such as barrel distortion, in an optical image.
In the preferred embodiment, the calibration image is placed over an imaging surface. For example, the calibration image may be a film with regularly spaced horizontal and vertical grid-lines etched onto the film. The imaging surface may be an imaging screen, such as a fluorescent phosphor screen used in x-ray imaging. The image of the calibration image is captured by an imaging sensor, such as a CCD camera.
In the preferred embodiment, the cell size of a particular cell is determined. Because the optical lenses tend to be fairly flat in the center, the center most cell of the image has the least distortion. Therefore, preferably the center most cell of the image is selected for determining the cell size. Moreover, because the optical plane of the imaging sensor and the calibration image may not be properly aligned with each other, therefore, in the preferred embodiment, the determined cell size may then be corrected for rotational differences between the imaging sensor and the calibration image. The factor by which each pixel is to be stretched in the preferred embodiment to remove the barrel distortion in either the x direction or the y direction is then determined. The stretch factor is calculated so that each cell in the resultant image is of consistent size. The stretch factors may be stored in mapping files associated with a processor based system, such as a personal computer. Once calculated and stored, these stretch factors may be applied to subsequent images to correct optical distortion in such images quickly and efficiently.
Accordingly, it is a technical advantage of a preferred embodiment of the present invention, to remove optical distortion from digital images.
It is another technical advantage of a preferred embodiment of the present invention to quickly and efficiently correct optical distortion.
It is still another technical advantage of a preferred embodiment of the present invention to facilitate combining of images.
It is yet another technical advantage of a preferred embodiment of the present invention to provide stretch factors which may be stored on a computer system to correct optical distortion in digital images.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.